Commercial and industrial processes commonly use a system of control valves to control the flow of a process fluid through a process plant. Many process control valves, including threaded internal valves, regulators, or other control devices, require actuators to position a flow control element inside the valve in a desired position. The control element, in turn, controls the flow of a process fluid through the valve. In many applications, the actuator is mounted directly to the valve body and connects to a valve stem, which in turn is operatively coupled to the flow control element (e.g., a closure member). Depending on the type of valve, the valve actuator either rotates the valve stem or moves the valve stem in a linear direction.
In various applications, valve designers may desire to convert one type of motion into another type of motion. For example, some applications require the conversion of translational movement in one direction into translational movement in another direction. An example of one such application is an axial flow valve which requires movement generated by a power source external to a flow passage to be converted into axial movement of a component inside the flow passage. Still other examples are known in the industry.
FIG. 1 illustrates an example of a known axial flow valve 100. The axial flow valve 100 includes a flow passage 110 extending through a valve body 120. A closure member 130 is positioned inside the flow passage 110 to selectively open and close the flow passage 110. The closure member 130 is movable along a central axis of the flow passage 110 between a closed position preventing fluid flow through the flow passage 110 and an open position permitting fluid flow through the flow passage 110. Axial movement of the closure member 130 is provided by the interaction of a drive linear rack 140 and a driven linear rack 150 located within the flow passage 110. A housing 155 protects the drive rack 140 and the driven linear rack 150 from the fluid flowing through the flow passage 110. The drive rack 140 is moved by a linear actuator (not shown) external to flow passage 110. Gear teeth 160 of the drive rack 140 meshingly engages gear teeth 170 of the driven linear rack 150. Linear motion of the drive rack 140 causes the driven linear rack 150 to move along the axial direction.
Positioning the closure member 130 in the open position of FIG. 1 requires moving a rear end 175 of the driven linear rack 150 away from the drive rack 140. An enlarged rear end 180 of the housing 155 is needed to accommodate the rear end 175 of the driven linear rack 150 in this position. The enlarged rear end 180 of the housing 155 impacts the fluid flow in the flow passage 110 and also increases the costs and complexity of manufacturing the housing 155 and/or the valve body 120. Another consequence of the arrangement shown in FIG. 1 is that axial motion is lost to the racks 160, 170, and the friction created in this arrangement causes a need for larger linear actuators. As such, the lengths of the racks 140, 150 are extended in order to achieve the desired travel distance of the closure member 130. This further increases the size of the housing 155 and the valve body 120.